The invention relates to the spectroscopic evaluation of isotope ratios in a gas sample. More particularly, the invention relates to an infrared spectrometer with low spectral resolution for the evaluation of isotope ratios of an atom in a compound of a gas sample. The spectroscopic isotope analyzer is suitable for medical diagnostics.
The evaluation of isotope ratios has applications in a variety of fields including chemistry, biology, geology and archeology. For example, 14C dating has been used in geology and archeology to evaluate the age of dead biological tissue. Also, the measurement of stable isotopes can be used in biology to study metabolic processes. The measurement of isotope ratios have also found use in medicine for the evaluation of disease, especially diseases of the gastrointestinal tract.
Several human diseases of the gastrointestinal tract, such as gastritis and peptic ulcers, have been found in recent studies to be closely associated with bacterial infection by Helicobacter pylori. An estimated 4.5 million people in the U.S. annually suffer from peptic and gastric ulcers, of which about 80% are thought to be associated with H. pylori. In addition, the World Health Organization believes that over 80% of the population in developing countries may be infected with H. pylori. Furthermore, H. pylori is considered to be a Class I carcinogen that increases an infected person""s risk of developing stomach cancer.
Blood tests can be used to detect factors associated with the infections, but blood tests do not indicate whether or not the infection is active. Metabolic activity of nonphotosynthetic cells generally involves the oxidation of organic compounds and the corresponding production of carbon dioxide. Thus, production of carbon dioxide is a direct indication of metabolic activity.
In a first aspect, the invention pertains to an infrared spectrometer for the evaluation of isotopic ratios in a gas sample, the spectrometer comprising:
a broad-band infrared light source, wherein light emitted by the light source proceeds along a light path;
a spectral selector placed along the light path, wherein the spectral selector selectively transmits a wavelength window of infrared light covering a range of infrared wavelengths, in which the wavelength window can be selected alternatively to overlap with a wavelength range primarily absorbed by a compound with a first isotope or by the compound with a second isotope;
a sample compartment for holding a gaseous sample, wherein the light path passes through the sample compartment, the sample compartment comprising a gas inlet and a gas outlet;
an infrared detector placed along the light path to receive infrared light after passing through the spectral selector and the sample compartment; and
a processor connected to receive output from the infrared detector, wherein the processor evaluates a quantity related to the ratio of isotopes.
In another aspect, the invention pertains to a method for determining a quantity related to the isotopic ratio of a compound in a gaseous sample, the method comprising:
directing broad band infrared light through a gas sample;
selecting a first wavelength window by spectrally separating light from the broad band light source, wherein the first wavelength window overlaps with a wavelength range primarily absorbed by the compound with a first isotope;
selecting a second wavelength window by spectrally separating light from the broad band light source, wherein the second wavelength window overlaps with a wavelength range primarily absorbed by the compound with a. second isotope;
detecting infrared light in the first wavelength window following passage through the gas sample to obtain a first value of detected infrared light;
detecting infrared light in the second wavelength. window following passage through the gas sample to obtain a second value of detected infrared light; and
evaluating a value related to the isotopic ratio of a compound in a gaseous sample from the values of detected infrared light.
In addition, the invention pertains to a multipass optical cell comprising:
a field mirror having a focal length and a center axis;
a two segment objective mirror generally facing the field mirror wherein the two segments are displaced from each other to move their respective focal points away from each other; and
a prismatic mirror displaced from the field mirror by less than about 20 percent of the focal length of the field mirror, wherein the edge of intersecting faces of the prismatic mirror is generally oriented toward the objective mirror and wherein the plane bisecting the two faces of the prismatic mirror pass through the two segments of the objective mirror.